Re: [PATCH v4 1/2] mm: refactor __vunmap() to avoid duplicated call to find_vm_area()

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 18, 2019 at 03:24:31PM -0700, Andrew Morton wrote:
> On Thu, 18 Apr 2019 04:18:34 -0700 Matthew Wilcox <willy@infradead.org> wrote:
> 
> > On Wed, Apr 17, 2019 at 02:58:27PM -0700, Andrew Morton wrote:
> > > On Wed, 17 Apr 2019 12:40:01 -0700 Roman Gushchin <guroan@gmail.com> wrote:
> > > > +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> > > > +{
> > > > +	struct vm_struct *vm = va->vm;
> > > > +
> > > > +	might_sleep();
> > > 
> > > Where might __remove_vm_area() sleep?
> > > 
> > > >From a quick scan I'm only seeing vfree(), and that has the
> > > might_sleep_if(!in_interrupt()).
> > > 
> > > So perhaps we can remove this...
> > 
> > See commit 5803ed292e63 ("mm: mark all calls into the vmalloc subsystem as potentially sleeping")
> > 
> > It looks like the intent is to unconditionally check might_sleep() at
> > the entry points to the vmalloc code, rather than only catch them in
> > the occasional place where it happens to go wrong.
> 
> afaict, vfree() will only do a mutex_trylock() in
> try_purge_vmap_area_lazy().  So does vfree actually sleep in any
> situation?  Whether or not local interrupts are enabled?

IIRC, the original problem that used to prohibit vfree() in interrupts
was the use of spinlocks that were used in a lot of places by plain
spin_lock().  I'm not sure it could actually sleep in anything not
too ancient...